Journal of Chromatography A

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1 Accepted Manuscript Title: System map for the ionic liquid stationary phase tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide for gas chromatography Authors: Nicole Lenca, Colin F. Poole PII: S (17) DOI: Reference: CHROMA To appear in: Journal of Chromatography A Received date: Revised date: Accepted date: Please cite this article as: Nicole Lenca, Colin F.Poole, System map for the ionic liquid stationary phase tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide for gas chromatography, Journal of Chromatography A This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 1 System map for the ionic liquid stationary phase tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide for gas chromatography Address for correspondence: Prof. C. F. Poole Highlights By Nicole Lenca and Colin F. Poole* Department of Chemistry Wayne State University Detroit MI USA Rm 185 Chemistry Wayne State University Detroit, MI USA Tel: Fax: System map for SLB-IL76 for the temperature range from C Intermolecular interactions responsible for retention are characterized Hydrogen-bond acidity is not clearly connected with the amide groups of the cation The SLB-IL76 column has complementary separation properties to HP-88 and similar bis(cyanopropyl)siloxane stationary phases Abstract The solvation parameter model is used to construct a system map for the retention of volatile organic compounds on the ionic liquid stationary phase tri(tripropypphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide (SLB- IL76) over the temperature range C. The SLB-IL76 stationary phase is moderately cohesive and strongly dipolar/polarizable and hydrogen-bond basic but only a weak hydrogenbond acid. Electron lone pair interactions are weak and make only a minor contribution to the

3 2 retention mechanism. The separation properties of SLB-IL76 highlight the difficulty of designing new stationary phases from ion structures as the presence of amide groups in the cation don t seem to contribute significantly to the hydrogen-bond acidity of SLB-IL76. The separation properties of SLB-IL76 are closest to the bis(polycyanopropyl)siloxane stationary phases with a high percentage of bis(cyanopropyl)siloxane monomer and could be used in method development when a stationary phase with similar gross retention characteristics but different selectivity is required. Keywords: Gas chromatography; Retention; Selectivity; Solvation parameter model; System map; Stationary phase characterization 1. Introduction Historically many liquids have been evaluated as stationary phases for gas chromatography but few survived as most are incapable of forming stable films on fused-silica surfaces resistant to solvents and high temperatures as well as possessing favorable diffusion properties to facilitate rapid mass transfer [1, 2]. Those available as pre-coated columns are dominated by poly(siloxanes) synthesized from different monomers affording a limited range of selectivity and poly(ethylene glycols). The stringent requirements for a useful stationary phase became a barrier for stationary phase development in the 1990s with new columns being mainly application-specific columns employing conventional stationary phases with an optimized composition or phase ratio for a particular application. The desire to access a wider selectivity space than available using conventional stationary phases became possible with the development of ionic liquids with suitable properties as stationary phases for open-tubular columns at the turn of the last century stationary [3, 4]. Ionic liquids are novel organic solvents composed entirely of ions. Favorable properties for gas chromatography include the virtual absence of vapor pressure, high viscosity and a moderate surface tension facilitating film formation on fused-silica surfaces, moderate cohesion and strong polar interactions allowing retention of a wide range of compounds, and the potential to design new stationary phases with different retention properties by exploiting the diversity of available ion structures [5, 6]. They complement the separation properties of conventional poly(siloxane) and poly(ethylene glycol) stationary phases by extending the column temperature operating limit and by facilitating separations that require a different selectivity to those provided by conventional stationary phases.

4 3 Sharma et al. [7] synthesized a new type of trigonal tricationic ionic liquids with favorable properties for gas chromatography that were subsequently evaluated as stationary phases by Payagala et al. [8]. Pre-coated columns of the most promising of these trigonal tricationic ionic liquids, tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide, Figure 1, became available in 2009 (SLB-IL76). Practical applications of SLB-IL76 include the separation of long-chain fatty acid methyl esters with different chain lengths, chain branching and degree of unsaturation [9] and congener-selective separation of polychlorinated dibenzodioxins and dibenzofurans [10]. The polarity number assigned to SLB-IL76 would suggest that it has similar separation characteristics to the polar stationary phase poly(biscyanopropylcyanopropylphenylsiloxane) containing about 80% bis(cyanopropylsiloxane) monomer [11]. The polarity number is defined as the sum of the retention index differences for the first five McReynolds prototypical compounds (benzene, 2- pentanone, 1-nitropropane, n-butanol and pyridine) at 120 C with squalane as a reference stationary phase. The scale is normalized such that the ionic liquid stationary phase 1,9-di(3- vinylimidazolium)nonane bis(trifluoromethylsulfonyl)imide has a polarity number of 100. However, it is doubtful if the McReynolds system of stationary phase constants provides useful information for stationary phase selection in method development [12] or whether a single value scale like the polarity number based on the same principles as the McReynolds system of phase constants would be any better [13]. The solvation parameter model provides an alternative approach to the system of McReynolds phase constants to characterize the retention properties of stationary phases for gas chromatography [14-17]. System maps derived from the system constants of the solvation parameter model afford insight into the variation of column selectivity with temperature as a continuous variable [13-15, 18, 19]. The solvation parameter model in the form suitable for characterizing retention in gas chromatography is set out below log k = c + ee +ss + aa + bb +ll (1) where k is the retention factor and e, s, a, b, and l are system constants that describe the complementary interactions of the stationary phase with the solute descriptors (E, S, A, B, L). The solute descriptors are defined as the excess molar refraction E, dipolarity/polarizability S, hydrogen-bond acidity A, hydrogen-bond basicity B, and the gas-liquid partition constant on

5 4 hexadecane at 25 C L [20-22]. The system constants are determined from the experimental retention factors for a collection of diverse compounds with known descriptor values by multiple linear regression analysis. The identity of the test compounds for column evaluation is less important than the experimental design employed. The test compounds are selected to provide accessible retention properties, to cover a wide descriptor space with roughly even occupancy, and with low cross-correlation between descriptors.. The system constants for a number of ionic liquid stationary phases are summarized in [5, 6, 17]. Payagala et al. determined the system constants for a self-made open tubular column prepared from tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide at 70 and 100 C [8]. Rodriguez-Sanchez et al. [23] determined the system constants for a commercial version of this stationary phase at C. At 100 C, where a comparison of the system constants is possible, the two studies show good agreement for the contribution of dipole-type interactions to the retention mechanism but predict different properties for hydrogen-bonding interactions. This is of interest for the design of new stationary phases since the widely used poly(siloxane) and poly(ethylene glycol) stationary phases have a wide range of hydrogen-bond basicity but none are hydrogen-bond acids [18, 24]. A number of ionic liquids have been identified as weak hydrogen-bond acids extending the selectivity space for separations by gas chromatography [5, 6, 17]. The central core of the tri(tripropylphosphoniumhexanamido)triethylamine stationary phase, Figure 1, contains an amide group in each of the three linker arms of the cation potentially contributing to its polarity and hydrogen-bond acidity. Previous attempts to design ionic liquid stationary phases with hydrogen-bond acidity employing alkylsulfonate anions containing hydroxyl, amide, or amine groups were unsuccessful [24-26]. The problem seems to be that ionic liquids which are simultaneously strong hydrogen-bond acids and hydrogen-bond bases prefer to form internal hydrogen-bonded complexes rather than form solute-solvent hydrogen-bonds. Thus, these ionic liquids were classified as non-hydrogen bond acids with respect to their separation properties by gas chromatography. A question we wished to answer is whether the restricted flexibility of the amide-containing arms in the tri(tripropylphosphoniumhexanamido)triethylamine cation would be more favorable for promotion of solute-solvent hydrogen-bonds with the stationary phase acting as a hydrogen-bond acid. This is in addition to providing a system map for SLB-IL76 for

6 5 its full operating temperature range for comparison with a database of system constants for nonionic and ionic stationary phases [6, 13, 18, 27, 28] 2. Experimental 2.1 Materials Common chemicals used for column characterization were of the highest purity available and obtained from several sources. The 30 m x 0.25 mm internal diameter SLB-IL76 opentubular column, 0.20 m film thickness, was obtained from Supelco (Bellefonte, PA, USA). 2.2 Instrumentation Retention factor measurements were made with an Agilent Technologies (Palo Alto, CA, USA) HP6890 gas chromatograph fitted with a split/splitless injector and flame ionization detector using Chemstation software (rev. B04.01) for data acquisition. Nitrogen was used as the carrier gas at a constant flow rate of 1.0 ml/min. The split ratio was generally 30:1 but varied to control peak detection, septum purge 1 ml/min, inlet temperature 280 C and detector temperature 280 C. Methane was used to determine the column hold-up time. Retention factors were measured at 20 C intervals over the temperature range C for varied compounds, selected so as to obtain experimentally accessible retention factors and statistically meaningful retention models (see section 2.3). 2.3 Calculations Multiple linear regression analysis and statistical calculations were performed on a Dell Optiplex 9020 computer (Austin, TX, USA) using the program PASW Statistics 24 (SPSS, Chicago, IL, USA). The core collection of compounds and their descriptor values for column characterization are given in [27] with additional values for polycylic aromatic compounds [29], flavor and fragrance compounds [30] and plasticizers [31] added to ensure adequate cover of the descriptor space and retention factor range. The selected compounds cover the descriptor space E = 0-3.0, S= 0-2.0, A = , B = , and L = The procedure for compound selection is detailed in ref. [27]. 3 Results and discussion The system constants at 20 C intervals for the SLB-IL76 column over the temperature range C are summarized in Table 1. The fit of the retention factors to the solvation

7 6 parameter model at all temperatures are satisfactory with multiple correlation coefficients > 0.994, standard error of the estimate < 0.068, and Fisher statistic > 978 and n To assist in the general interpretation of the system constants the results are plotted as a system map in Figure 2. The SLB-IL76 column is moderately cohesive, strongly dipolar/polarizable and hydrogen-bond basic but a weak hydrogen-bond acid. Electron lone pair interactions are weak and make only a minor contribution to the retention mechanism. It is noteworthy that the interactions of a dipole-type and hydrogen-bond basicity of the ionic liquid persist to the maximum operating temperature for the stationary phase with a rather shallow temperature dependence. The s system constant changes from about 1.7 to 1.2 over a 160 C temperature range and the a system constant from about 1.7 to 0.9 for the same temperature range. Thus, these interactions remain an important contribution to the retention mechanism at all temperatures. The l system constant varies from about 0.47 to 0.17 over the same temperature range and are similar in magnitude and direction to typical results for conventional polar stationary phases [18, 19]. From this we can deduce that the cohesive energy of the ionic liquid stationary phase is similar to conventional polar stationary phases and SLB-IL76 is expected to have favorable retention properties for volatile organic compounds. The SLB-IL76 column exhibits only weak hydrogen-bond acidity at temperatures < 220 C with the b system constant varying from nearly 0 to about 0.44 over a 140 C temperature range. The hydrogen-bond acidity for SLB-IL76 can be compared to three other pre-coated ionic liquid stationary phases with a related structure, SLB-IL60 [13], SLB-IL61 [28] and SLB-IL100 [27]. A plot of the b system constant against temperature for the compared phases is shown in Figure 3. It provides an illustration of how difficult it is to design ionic liquids with specific solvation properties simply from ion structures. The 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide (SLB-IL60) and di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethylsulfonate (SLB-IL61) stationary phases are not expected to be hydrogen-bond acids since they lack typical hydrogen-bond acid functional groups while the tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide (SLB-IL76) would be expected to be a reasonably strong hydrogen-bond acid on account of the amide groups in the cation. Just the opposite behavior is indicated by Figure 3 with the SLB-IL76 column being generally the weakest hydrogen-bond acid of the four columns. The imidazolium cations of 1,9-Di(3-vinylimidazolium)none

8 7 bis(trifluoromethylsulfonyl)imide (SLB-IL100) are weak hydrogen-bond acids and might be accepted as the source of this property. Comparing SLB-IL60 and SLB-IL61 with SLB-IL76 would suggest that the contribution of the amide groups to the hydrogen-bond acidity of SLB- IL76 is weak at best. The hydrogen-bond acidity for the three columns probably results in the main from a different source. This was speculated to result from the interaction of the anion with the carbon chain of the cation increasing the hydrogen-bond acidity of the methylene groups due to proximity and restricted flexibility in the structure of the ionic liquids [13, 28]. This remains speculation for the time being as the bis(trifluoromethylsulfonyl)imine is a weak nucleophile and would not be an obvious candidate for such an interaction in the absence of the observed results. In ref. [13, 28] it is argued that the hydrogen-bond acidity of SLB-IL60 and SLB-IL61 is not likely due to adsorption on the surface of the fused-silica capillary wall. Whether adequately explained or not, the presence of weak hydrogen-bond acid interactions makes the ionic liquid stationary phases of unique interest for method development in gas chromatography since except for hydrocarbons virtually all other compounds are hydrogen-bond bases and their separation properties are expected to be affected by the hydrogen-bond acidity of the ionic liquids in a way not observed for conventional non-ionic stationary phases, which are all non-hydrogen-bond acids [18, 24]. We can compare our results for SLB-IL76 with those reported by Rodriguez-Sanchez et al. [23] and Payagala et al. [8] for tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide stationary phases. There is good agreement with the results of Rodriguez-Sanchez et al. for the temperature range C for an SLB-IL76 column. The average difference in system constants (calculated as the system constants from this study the system constants given in ref. [23] for the same four temperatures) is e = (SD= 0.003), s = (SD = 0.006), a = (SD=0.009), b = (SD=0.041) and l = (SD = 0.004) where SD = standard deviation with n = 4. Given that the results are for different columns, different compounds and different instruments the small and generally systematic differences are acceptable when viewed against the magnitude of the system constants. Both studies support the same general retention mechanisms for the temperature range of the comparison. For 100 C the difference in the system constants reported here and those of Payagala et al. [8] are e = -0.03, s = -0.05, a = -0.58, b = 0.20 and l = The results of Payagala et al. [8] refer to a self-made column and a rather small number of test compounds.

9 8 Given these additional differences there is good agreement for the e, s and l system constants but poor agreement for the hydrogen-bonding properties of the stationary phase (a and b system constants). It is not possible to assign a reason for these differences beyond the possible contributing factors indicated above. The retention properties of SLB-IL76 can be compared with conventional non-ionic stationary phases and the ionic liquid stationary phases SLB-IL60, SLB-IL61 and SLB-IL100 using principal component analysis with the system constants as variables, Figure 4. To simplify this figure stationary phases with duplicate properties were removed to better illustrate the occupancy of the selectivity space. The ionic liquid stationary phases occupy an otherwise empty region of the selectivity space indicating that their separation properties are not duplicated by the non-ionic stationary phases. SLB-IL76 is the closest member of the ionic liquid stationary phases to the conventional stationary phases with properties similar to the group of poly(cyanopropylsiloxane) phases with a high percentage of bis(cyanopropyl)siloxane monomer. It s closest neighbor in the selectivity space is HP-88, a copolymer of bis(cyanopropyl)siloxane and methylsilarylene monomers in a ratio of approximately 9:1 [32] The SPB-IL76 stationary phase is slightly more cohesive, less dipolar/polarizable and a weaker hydrogen-bond base than HP-88 whereas SLB-Il76 is a weak hydrogen-bond acid, while HP-88 has no hydrogen-bond acidity. Conclusions SLB-IL76 is a polar stationary phase with complementary separation properties to conventional poly(siloxane) and poly(ethylene glycol) stationary phases. It shares some properties in common with bis(cyanopropyl)siloxane stationary phases with a high percentage of bis(cyanopropylsiloxane) monomer but with small differences in selectivity. One important difference is that SLB-IL76 is a weak hydrogen-bond acid while the bis(cyanopropyl)siloxanes are not. By comparison to other ionic liquid stationary phases there is a lack of evidence that the presence of amide groups in the cation structure contribute in a significant way to the hydrogenbonding acidity of SLB-IL76. This serves to illustrate how difficult it is to set about designing stationary phases with specific solvation properties from consideration of their ion structures. All ionic liquids appear to be polar solvents but other factors beyond the type of functional groups introduce into their structure need to be considered to arrive at a global picture of their retention properties.

10 9 References [1] C. F. Poole, The essence of chromatography, Elsevier, Amsterdam, 2003, p [2] K. Grob, Making and manipulating capillary columns for gas chromatography, Huethig, Heidelberg, 1986 [3] Z-Q Tan, J-F. Liu, L. Pang, Advances in analytical chemistry using the unique properties of ionic liquids, Trends Anal. Chem. 39 (2012) [4] T.D. Ho, C. Zhang, L.W. Hatano, J.L. Anderson. Ionic liquids in analytical chemistry: fundamentals, advances, and perspectives, Anal. Chem. 86 (2014) [5] C.F. Poole, S.K. Poole, Ionic liquid stationary phases for gas chromatography, J. Sep. Sci. 34 (2011) [6] C.F. Poole, N. Lenca, Gas chromatography on wall-coated open-tubular columns with ionic liquid stationary phases, J. Chromatogr. A 1357 (2014) [7] P.S. Sharma, T. Payagala, E. Wanigasekara, A.B. Wijeratne, J. Huang, D.W. Armstrong, Trigonal tricationic liquids: molecular engineering of trications to control physicochemical properties, Chem. Mat. 20 (2008) [8] T. Payagala, Y. Zhang, E. Wanigasekara, K. Huang, Z.S. Breitbach, P.S. Sharma, L.M. Sidisky, D.W. Armstrong, Trigonal tricationic ionic liquids: A generation of gas chromatographic stationary phases, Anal. Chem. 81 (2009) [9] A.X. Zeng, S.T. Chun, Y. Nolvachai, C. Culsing, L.M. Sidisky, P.J. Marriott, Characterization of capillary ionic liquid columns for gas chromatography-mass spectrometry analysis of fatty acid methyl esters, Anal. Chim. Acta 803 (2013) [10] L.D,P. Liljelind, J. Zhang, P. Haglund, Comprehensive profiling of 136 tetra- to octapolychlorinated dibenzo-p-dioxins and dibenzofurans using ionic liquid columns and column combinations, J. Chromatogr. A 1311 (2013) [11] C. Ragonese, D. Sciarrone, P.Q. Tranchida, P. Dugo, L. Mondello, Evaluation of a medium-polarity ionic liquid stationary phase in the analysis of flavor and fragrance compounds, Anal. Chem. 83 (2011) [12] B.R. Kersten, C.F. Poole, K.G. Furton, Ambiguities in the determination of McReynolds stationary phase constants, J. Chromatogr. 411 (1987) [13] N. Lenca, C.F. Poole, A system map for the ionic liquid stationary phase 1,12- di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide for gas chromatography, J. Chromatogr. A (submitted). [14] M.H. Abraham, C.F. Poole, S.K. Poole, Classification of stationary phases and other materials by gas chromatography, J. Chromatogr. A842 (1999)

11 10 [15] C.F. Poole, S.K. Poole, Column selectivity from the perspective of the solvation parameter model, J. Chromatogr. A 965 (2002) [16] M.F. Vitha, P.W. Carr. The chemical interpretation and practice of linear solvation energy relationships in chromatography. J. Chromatogr. A 1126 (2006) [17] C. Yao, J.L. Anderson, Retention characteristics of organic compounds on molten salt and ionic liquid-based gas chromatography stationary phases (review), J. Chromatogr. A 1216 (2009) [18] C.F. Poole, S.K. Poole, Separation characteristics of wall-coated open-tubular columns for gas chromatography, J. Chromatogr. A 1184 (2009) [19] S.N. Atapattu, K. Eggers, C.F. Poole, W. Kiridena, W.W. Koziol, Extension of the system constants database for open-tubular columns: System maps at low and intermediate temperatures for four new columns, J. Chromatogr. A 1216 (2009) [20] M.H. Abraham, A. Ibrahim, A.M. Zissmos. Determination of sets of solute descriptors from chromatographic measurements. J. Chromatogr. A 1037 (2004) [21] C.F. Poole, S.N. Atapattu, S.K. Poole, A.N. Bell. Determination of solute descriptors by chromatographic methods. Anal. Chim. Acta 652 (2009) [22] C. F. Poole, T. C. Ariyasena and N. Lenca. Estimation of the environmental properties of compounds from chromatographic measurements and the solvation parameter model. J. Chromatogr. A 1317 (2013) [23] S. Rodriguez-Sanchez, P. Galindo-Iranzo, A.C. Soria, M.L. Sanz, J.E. Quintanilla-Lopez, R. Lebron-Aguilar. Characterization by the solvation parameter model of the retention properties of commercial ionic liquid columns for gas chromatography, J. Chromatogr. A 1326 (2014) [24] S.D. Martin, C.F. Poole, M.H. Abraham, Synthesis and gas chromatographic evaluation of a high-temperature hydrogen bond acid stationary phase, J. Chromatogr. A 805 (1998) [25] R.M. Pomaville, C.F. Poole. Solute-solvent interactions in liquid tetrabutylammonium sulfonate salts studied by gas chromatography. Anal. Chem. 60 (1988) [26] S. K. Poole and C. F. Poole. Chemometric evaluation of the solvent properties of liquid organic salts. Analyst 120 (1995) [27] N. Lenca, C.F. Poole. A system map for the ionic liquid stationary phase 1,9-di(3- vinylimidazolium)nonane bis(trifluoromethylsulfonyl)imide. Chromatographia 78 (2014)

12 11 [28] N. Lenca, C. F. Poole. A system maps for the ionic liquid stationary phase 1,12- di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate for gas chromatography. J. Chromatogr. A (submitted) [29] T.C. Ariyasena, C.F. Poole. Determination of descriptors for polycyclic aromatic hydrocarbons and related compounds by chromatographic methods and liquid-liquid partition in totally organic biphasic systems. J. Chromatogr. A 1361 (2014) [30] T. Karunasekara, C.F. Poole, Determination of descriptors for fragrance compounds by gas chromatography and liquid-liquid partition. J. Chromatogr. A 1235 (2012) [31] T. Karunasekara, S.N. Atapattu, C.F. Poole. Determination of descriptors for plasticizers by chromatography and liquid-liquid partition. Chromatographia 75 (2012) [32] W. Kiridena, C.C. Patchett, W.W. Koziol, C.F. Poole. System constants for the bis(cyanopropylsiloxane)-co-methylsilarylene HP-88 and poly(siloxane) Rtx-440 stationary phases. J. Chromatogr. A 1081 (2005)

13 12 Figure legends Figure 1 Figure 2 Figure 3 Figure 4 Structure for SLB-IL76, tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide A system map for the ionic liquid column SLB-IL76. Plot of the b system constant for four ionic liquid stationary phases as a function of temperature. The columns are labeled using the abbreviations IL-60 for 1,12- di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide SPB- IL60, IL-61 for 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate SLB-IL61, IL76 for tri(tripropylphosphoniumhexanamido)triethylamine bis(trifluoromethylsulfonyl)imide SPB-IL76 and IL100 for 1,9-di(3- vinylimidazolium)nonane bis(trifluoromethylsulfonyl)imide SLB-IL100. Score plot for principal component factor analysis (varimax rotation) of the system constants at 140 C for 32 pre-coated open-tubular columns. The first two principal components describe 83.6% of the variance. Model for the loadings PC- 1 = 0.81 b s a 0.08 e 0.94 l and PC-2 = 0.91 e a s 0.07 b 0.21 l. The open circles are various low-polarity poly(siloxane) stationary phases prepared from mixtures of dimethylsiloxane, methylphenylsiloxane and diphenylsiloxane monomers. The groups labeled PMTS are poly(siloxane) stationary phases containing different amounts of 3- trifluoropropylsiloxane monomers; PCPS poly(siloxane) stationary phases containing different amounts of bis(3-cyanopropyl)siloxane monomer (BPX90, SP-2340, HP-88, DB-23 and DB-225), and PEG poly(ethylene glycol) stationary phases. The ionic liquid stationary phases are identified using the abbreviations introduced in Figure 3. Table 1 System constants for the SLB-IL76 column Temper- System constant Statistics* ature ( C) c e s a b l r SE F n

14 (0.052) (0.024) (0.041) (0.083) (0.049) (0.008) (0.044) (0.022) (0.033) (0.050) (0.038) (0.007) (0.038) (0.019) (0.030) (0.035) (0.032) (0.005) (0.033) (0.016) (0.026) (0.030) (0.026) (0.004) (0.027) (0.014) (0.021) (0.026) (0.021) (0.003) (0.025) (0.012) (0.017) (0.024) (0.019) (0.003) (0.026) (0.012) (0.018) (0.025) (0.020) (0.003) (0.025) (0.011) (0.015) (0.022) (0.016) (0.003) (0.024) (0.010) (0.015) (0.023) (0.002) * r = multiple correlation coefficient, SE = standard error in the estimate, F = Fisher statistic, and n = number of compounds with retention factor values included in the model. The numbers in parenthesis are the standard deviations for the individual system constants.

15 Figure 1

16 System Constants 2 0 e l s a b -2 c Temperature ( C) Figure 2

17 b System Constant 0.8 IL IL61 IL60 IL Temperature ( C) Figure 3

18 IL100 IL60 IL61 IL76 PMTS PCPS PEG Figure 4